The invention relates to an electrical circuit with a nanostructure and a method for producing a contact-connection of a nanostructure.
Conventional silicon microelectronics will reach its limits as miniaturization advances further. In particular the development of increasingly smaller and more densely arranged transistors of, in the meantime, hundreds of millions of transistors per chip will be subject to fundamental physical problems and limitations in the next ten years. If structural dimensions fall below approximately 80 nm, the components are influenced by quantum effects in a disturbing manner, and they are dominated by quantum effects at dimensions of below about 30 nm. The increasing integration density of the components on a chip also leads to a dramatic increase in the waste heat.
A known possible successor technology to follow conventional semiconductor electronics is nanostructures such as, for example, nanotubes, in particular carbon nanotubes, and nanorods, also called nanowires.
A carbon nanotube is a single-wall or multiwall, tubular carbon compound. In the case of multiwall nanotubes, at least one inner nanotube is coaxially surrounded by an outer nanotube. Single-wall nanotubes typically have diameters of 1 nm, while the length of a nanotube may amount to hundreds of nm. The ends of a nanotube are often terminated with in each case half a fullerene molecule. Nanotubes can be produced by depositing a catalyst material layer, for example composed of iron, cobalt or nickel, on a substrate and, on said catalyst material layer, growing carbon nanotubes on the catalyst material layer using a CVD method (“chemical vapor deposition”) by means of introducing a carbon-containing material (for example acetylene) into the method chamber. On account of the good electrical conductivity of carbon nanotubes and also on account of the adjustability of said conductivity, for example by means of applying an external electric field or by means of doping the nanotubes with potassium, for example, nanotubes are suitable for a large number of applications, in particular in the electrical coupling technology in integrated circuits, for components in microelectronics and also as electron emitters.
Field effect transistors are required for many integrated components in silicon microelectronics. Carbon nanotubes can be used for forming such a field effect transistor, whereby a so-called CNT-FET (“carbon nanotube field effect transistor”) is formed R. Martel et al., “Ambipolar Electrical Transport in Semiconducting Single-Wall Carbon Nanotubes”, Physical Review Letters, Vol. 87 No. 25 (2001) Art. 265805, S. Heinze et al., “Carbon Nanotubes as Schottky Barrier Transistors”, Physical Review Letters, Vol. 89 No. 10 (2002) Art. 106801. For this purpose, by way of example, a nanotube is formed in planar fashion on a dielectric layer on a conductive substrate and contact-connected. The conductivity of the carbon nanotube is controlled by means of a suitable electrical voltage applied to the conductive substrate, with the result that the electric current flow through the nanotube, clearly the electric current flow between the source/drain terminals of the CNT-FET, can be controlled by means of applying a voltage to the conductive substrate.
One problem that can occur in the case of such a CNT-FET, however, is the relatively high contact resistance that forms between the channel region of the CNT-FET and the source/drain terminals and is produced at least partly by the formation of a Schottky barrier.
The formation of such a Schottky barrier is disadvantageous particularly in a second field of application in which CNTs can be used. In this case, CNTs are used as so-called vias or interconnects, that is to say as connecting lines between interconnects or electrical subcircuits situated on different planes of an electrical circuit, or else vias in so-called 3D integration, that is to say a suitable formation one above another or stacking one above another of the individual components or system units of the integrated circuit. It is thereby possible to increase the packing density and thus to reduce the space required in a plane. A further application of 3D integration is the stacking one above another of different integrated circuits of so-called systems which are intended to be integrated on a common chip, whereby a so-called system-on-chip (SoC) is formed.
In order to solve the problem of the high contact resistance, various metals are tested as contact electrodes in order to reduce the contact resistance with respect to CNTs. By way of example, in the case of single wall carbon nanotubes (SWCNT) having a large diameter, so-called transparent contacts, that is to say ohmic contacts, were obtained by producing the contacts with palladium A. Javey et al., “Ballistic Carbon Nanotube Field-Effect Transistors”, Nature, Vol. 424 (August 2003) pp. 654-657, but such SWCNTs having a large diameter are only poorly suited to being used to form a CNT-FET. Further possibilities for reducing the contact resistance are, on the one hand, the formation of carbides by means of carbide-forming agents, for example titanium, and, on the other hand, the enlargement of the contact area, but only very few materials are known which can function as carbide-forming agents, and an enlargement of the contact area goes against the desire in microelectronics for miniaturizing all components.
Graham et al., “Towards the integration of carbon nanotubes in microelectronics” in Diamond and Related Materials, Vol. 13, April-August 2004, pp. 1296-1300 describes general concepts for the integration of carbon nanotubes in microelectronics, inter alia a concept for producing carbon nanotube vias, nickel being proposed for the contact-connection of the carbon nanotubes.
DE 198 56 295 C2 describes a gate-electrode-forming carbon layer of a pH-sensitive field effect transistor.
EP 1 496 554 A2 describes a thin-film transistor in which a threshold-voltage-controlling film is provided between a gate-insulating film and an organic semiconductor film, with the result that the threshold voltage can be controlled in a simple manner.